Multi-user MIMO systems and methods

ABSTRACT

A method and system are provided for scheduling data transmission in a Multiple-Input Multiple-Output (MIMO) system. The MIMO system may comprise at least one MIMO transmitter and at least one MIMO receiver. Feedback from one or more receivers may be used by a transmitter to improve quality, capacity, and scheduling in MIMO communication systems. The method may include generating or receiving information pertaining to a MIMO channel metric and information pertaining to a Channel Quality Indicator (CQI) in respect of a transmitted signal; and sending a next transmission to a receiver using a MIMO mode selected in accordance with the information pertaining to the MIMO channel metric, and an adaptive coding and modulation selected in accordance with the information pertaining to the CQI.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/360,600 filed on Nov. 23, 2016 entitled, “Multi-User MIMO Systems andMethods”, which is a continuation of U.S. Pat. No. 9,538,408 issued onJan. 3, 2017 entitled “Multi-User MIMO Systems and Methods”, which iscontinuation of U.S. Pat. No. 9,301,174 issued on Mar. 29, 2016 entitled“Multi-User MIMO Systems and Methods”, which is a continuation of U.S.Pat. No. 8,611,454 issued on Dec. 17, 2013 entitled “Multi-User MIMOSystems and Methods”, which is a continuation of U.S. Pat. No. 8,284,852issued on Oct. 9, 2012 entitled “Multi-User MIMO Systems and Methods”,which is a continuation of U.S. Pat. No. 8,054,898 issued on Nov. 8,2011 entitled “Multi-User MIMO Systems and Methods”, which is a filingunder 35 U.S.C. § 371 of International Application No. PCT/CA2006/001665filed Oct. 12, 2006 entitled “Multi-User MIMO Systems and Methods”,claiming priority to U.S. Provisional Application No. 60/725,951 filedOct. 12, 2005, all of which are incorporated by reference herein as ifreproduced in their entirety.

TECHNICAL FIELD

The present application relates generally to communication systems ingeneral, and, more specifically, to MIMO (multiple-inputmultiple-output) communication systems.

BACKGROUND

In a MIMO communication system, a transmitter transmits data throughmultiple transmitting antenna (N_(T)) and a receiver receives datathrough multiple receiving antenna (N_(R)). The binary data to betransmitted is usually divided between the transmitting antennae. Eachreceiving antenna receives data from all the transmitting antennae, soif there are M transmitting antennae and N receiving antennae, then thesignal will propagate over M×N channels, each of which has its ownchannel response.

MIMO wireless communication systems are advantageous in that they enablethe capacity of the wireless link between the transmitter and receiverto be improved compared with previous systems in the respect that higherdata rates can be obtained. The multipath rich environment enablesmultiple orthogonal channels to be generated between the transmitter andreceiver. Data can then be transmitted over the air in parallel overthose channels, simultaneously and using the same bandwidth.Consequently, higher spectral efficiencies are achieved than withnon-MIMO systems.

SUMMARY

In some aspects of the present disclosure, a base station in amulti-user MIMO system selects a transmission method on the basis offeedback information received from a plurality of receivers.

In some aspects, the base station assigns a data rate and a MIMO modesuited to the channel quality for that user.

In some aspects, the present disclosure includes systems and methodswhich may compute MIMO channel metrics.

In some aspects, the present disclosure includes systems and methodswhich may include MIMO mode selection.

In some aspects, the present disclosure includes systems and methodswhich may assign/schedule MIMO user transmission and associated formatsin order to maximize MIMO communication capacity.

In some aspects, the present disclosure includes systems and methodswhich may be used in conjunction with OFDM sub-channels.

In some aspects, the present disclosure includes systems and methodswhich use uplink (UL) channel sounding where MIMO matrices may becalculated on the transmission side.

According to one broad aspect of the present disclosure, there isprovided a method comprising: i) generating a channel quality indicator(CQI) and a multiple-input multiple-output (MIMO) channel indication,the MIMO channel indication indicating if the MIMO channel isorthogonal; and ii) transmitting a composite metric based on the MIMOchannel indication and the CQI.

According to another broad aspect of the present disclosure, there isprovided a transceiver system comprising: i) a generator configured togenerate a channel quality indicator (CQI) and a multiple-inputmultiple-output (MIMO) channel indication, the MIMO channel indicationindicating if the MIMO channel is orthogonal; and ii) a transmitterconfigured to transmit a composite metric based on the MIMO channelindication and the CQI.

According to still another broad aspect of the present disclosure, thereis provided a method comprising: i) generating a channel qualityindicator (CQI) and a multiple-input multiple-output (MIMO) modeindication, the MIMO mode indication indicating a MIMO mode; and ii)transmitting a composite metric based on the MIMO mode indication andthe CQI.

According to yet another broad aspect of the present disclosure, thereis provided a transceiver system comprising: i) a generator configuredto generate a channel quality indicator (CQI) and a multiple-inputmultiple-output (MIMO) mode indication, the MIMO mode indicationindicating a MIMO mode; and ii) a transmitter configured to transmit acomposite metric based on the MIMO mode indication and the CQI.

Other aspects and features of the present disclosure will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of the specific embodiments of the presentdisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described in greater detail withreference to the accompanying diagrams, in which

FIG. 1 is a block diagram of a multi-user MIMO system in accordance withan embodiment of the present disclosure where channel measurements arecalculated on the receive side;

FIG. 2 is a block diagram of a multi-user MIMO system in accordance withan embodiment of present disclosure where channel measurements arecalculated on the transmission side;

FIG. 3A illustrates a lookup table for an embodiment of the presentdisclosure in which a Forward Error Correction (FEC) code, a modulationtype and a MIMO mode are selected based on a Channel Quality Indicator(CQI);

FIG. 3B illustrates a key table for FIGS. 3A and 4A;

FIG. 4A illustrates a lookup table for another embodiment of the presentdisclosure in which a Forward Error Correction (FEC) code and amodulation type are selected based on a CQI;

FIG. 4B illustrates a lookup table for a MIMO mode based on a MIMOindicator (MIMOI);

FIG. 5 is a diagram illustrating a graphical lookup for a MIMO modebased on a MIMOI and a CQI;

FIG. 6 is a plot of BLER (Block Error Rate) versus Signal-to-Noise Ratio(SNR) for an embodiment of the present disclosure;

FIG. 7 is a plot of BLER (Block Error Rate) versus SNR for anotherembodiment of the present disclosure;

FIG. 8A is a plot of Cumulative Distribution Function (CDF) versuscondition number of H^(H)H for an embodiment of the present disclosure;and

FIG. 8B is a plot of CDH versus H^(H)H for another embodiment of thepresent disclosure.

DETAILED DESCRIPTION

According to embodiments of the present disclosure, systems and methodsare provided which enhance the performance of communication channels ina communication system, to thereby improve, for example, thetransmission performance of multi-user MIMO communication systems.

In multi-user MIMO systems, a multi-data stream transmitter at a basetransceiver station (BTS) that provides communication services for acoverage area or cell in a wireless communication system transmitscommunication signals to a plurality of user terminals via multipleantennas. User terminals are also commonly referred to as MIMOreceivers, user equipment (UE), communication devices, and mobilestations, for instance. At a MIMO receiver side, multiple receiveantennas are employed for each user.

FIG. 1 is a block diagram of a multi-user MIMO system in accordance withone embodiment of the present disclosure. On the transmit side, thesystem of FIG. 1 includes a BTS 100 with an Adaptive Coding andModulation Module 102, user feedback module 108, and a pair of antennas110, 112. On the receive side, the system of FIG. 1 includes one or moreuser terminals 119, 125, 131 (three shown in the illustrated example)include respective MIMO receivers 118, 124, 130 each having a pair ofantennas 114,116, 120,122, and 126,128 respectively.

In user terminal 131, antennas 126, 128 are both connected to the MIMOreceiver 130 and to a MIMO channel module 132. MIMO channel module 132represents the real world radio propagation channel. MIMO receiver 130is connected to MIMO channel metric measurement module 134. MIMO channelmodule 132 is connected to CQI metric measurement module 136. Both MIMOchannel metric measurement module 134 and CQI metric measurement module136 are connected to composite feedback module 138. Composite feedbackmodule 138 forms part of the feedback path from the receive side to thetransmit side. Information regarding the MIMO channel metric and the CQImetric is transmitted from MIMO channel metric measurement module 134and CQI metric measurement module 136 respectively to composite feedbackmodule 138 which incorporates one or more lookup tables to determine aMIMO mode and data rate. The MIMO mode and data rate are fed back bycomposite feedback module 138 to BTS 100 by any convenientcommunications method, which may or may not comprise wirelesscommunications.

Each of user terminal 119 and user terminal 125 also include channelmeasurement modules as well (i.e. each have their own modules equivalentto MIMO channel module 132, MIMO channel metric measurement module 134,CQI metric measurement module 136, and composite feedback module 138).These modules, which are connected to each of MIMO receiver 118 and MIMOreceiver 124, have been intentionally omitted to simplify FIG. 1.

The system of FIG. 1 operates as follows. Pilot data is input intoadaptive coding and modulation module 102 where such pilot data isconverted into communication signals which may then be transmitted viathe antennas 110,112 from the BTS 100 to user terminals 119, 125 and131. In some embodiments, the pilots are inserted on each antenna in amanner that makes them distinguishable at a receiver. For example, forOFDM implementations, a respective set of sub-carrier and OFDM symboldurations can be employed for each antenna.

At MIMO receivers 118, 124, and 130, each of the antennas 114,116,120,126, and 126,128 receive the pilot signals transmitted from theantennas 110,112. MIMO receiver 130 processes the received signals toproduce separated layer signals which are fed to MIMO channel metricmeasurement module 134. The MIMO channel measurement metric measurementmodule 134 processes the received pilot data having regard to knowledgeof what the transmitted pilot data was, and produces a MIMO channelmetric. Specific examples of calculations which may be performed toassesses a MIMO channel metric are described below. MIMO channel module132 processes the received signal to produce MIMO channel stateinformation which is fed to the CQI measurement module 136. The CQImetric measurement module processes the received pilot data havingregard to knowledge of what the transmitted pilot data was, produces aCQI metric. CQI metrics are well known and may for example include CINR(carrier to interference and noise ratio), and the rank of the MIMOchannel.

The CQI metric is used as a basis for selecting a particular coding andmodulation. BTS 100 can adjust the modulation order and/or coding ratein accordance with the CQI metric. More particularly, the datatransmission rate can be increased, decreased, maintained at a constantlevel, or reduced to 0 bits/s. In a particular example, the CQI is CINRas indicated above, and each range of CINR is associated with arespective adaptive coding and modulation.

In some embodiments, MIMO receivers 118, 124 and 130 track the channelquality via the pilot symbols received and accumulate these qualitymeasurements over a period of time to produce the CQI.

In some embodiments, the feedback from user terminals 119, 125, 131 mayalso include information identifying the receiver's MIMO capability. Forexample, this might indicate a number of receive antennas, or the rankof the MIMO channel.

The MIMO channel metric is used to select a MIMO transmission mode to beused for transmitting to a particular user terminal. The particular MIMOmodes that are available are selected on an implementation specificbasis. Four examples of MIMO modes include beamforming, BLAST,space-time transmit diversity (STTD), and spatial multiplex, though thepresent disclosure is in no way limited to these MIMO modes and is infact applicable to all possible space-time mapping.

Those skilled in the art will appreciate that MIMO channel metricmeasurement and CQI metric measurement may be performed by a digitalsignal processor (DSP) or a general-purpose processor adapted to executesignal processing software, for example. Various techniques fordetermining such metric measurements will be apparent to those skilledin the art.

Both the MIMO channel metric and the CQI is transmitted to compositefeedback module 138 where one or more lookup tables may be used todetermine a composite metric used by BTS 100 to select a MIMO mode anddata rate. As used herein, “composite” can be equated to the “overall”quality of the channel matrix. The lookup carried out by compositefeedback module 138 is used for two purposes: (i) User terminal pairing,i.e. scheduling. The more orthogonal the channel, the larger the MIMOcapacity; and (ii) together with SNR, the lookup is used for MIMO modeand coding and modulation selection. With a higher SNR and compositemetric, spatial multiplexing and higher modulation and coding rates maybe selected. With a lower SNR and composite metric, transmit and lowermodulation and coding rates may be selected.

Note that the composite metric does not affect modulation and codingrates selection in transmit diversity, but it affects modulation andcoding rates selection in spatial multiplexing. This is because when thecomposite metric is low, more inter-layer interference will occur, andhence only lower modulation and coding rates are to be used.

The composite metric is then transmitted by composite feedback module138 to BTS 100 through user feedback module 108. With the compositemetric received from composite feedback module 138, a scheduler whichforms part of BTS 100 determines a MIMO transmission mode and amodulation and coding to be used for each MIMO receiver. In someembodiments, the BTS 100 indicates the transmission format to each MIMOreceiver.

In some embodiments, a two bit composite metric is used, with one bit ofthe composite metric being used to indicate the CQI, and one bit of thecomposite metric to indicate the MIMO mode, e.g. transmit diversity orspatial multiplexing. In the spatial multiplexing mode, one additionalbit can be used to indicate if the MIMO channel is orthogonal.

FIG. 2 is an alternative embodiment to that illustrated in FIG. 1. Inthis embodiment, channel metrics are measured at the transmit side 250rather than the receiver side 201. On the transmit side 250, the systemof FIG. 2 includes a BTS 200 with an Adaptive Coding and ModulationModule 202, user feedback module 208, and a pair of antennas 210,212. Onthe receive side 201, the system of FIG. 2 includes one or more userterminals 219, 225, 231 (three shown in the illustrated example) includerespective MIMO receivers 203, 204, 206 each having a pair of antennas214,216, 220,222, and 226,228 respectively.

In user terminals 219, 225, and 231, antennas 214,216, 220,222, and226,228 respectively are connected to MIMO receivers 203, 204, and 206which each perform UL channel sounding. In the case of Time DivisionDuplex (TDD), channel sounding is used to allow BTS 200 to performchannel measurements at the transmit side 250 rather than the receiverside 201. Information received from MIMO receivers 203, 204, and 206 isfed back through a feedback control channel to user feedback module 208at BTS 200 by any convenient communications method, which may or may notcomprise wireless communications.

User feedback module 208 is connected to both MIMO channel metricmeasurement module 234 and CQI metric measurement module 236. Both MIMOchannel metric measurement module 234 and CQI metric measurement module236 are connected to composite feedback module 238. Composite feedbackmodule 238 is connected to adaptive coding and modulation module 202.

Except for the fact that channel measurements are performed at thetransmit side 250 rather than the receive side 201, the operation of thesystem of FIG. 2 is otherwise similar to the operation of the system ofFIG. 1. The main difference is that through channel sounding, userterminals 219, 225, and 231 pass the burden of channel measurements andprocessing to BTS 200.

Of course, the systems of FIGS. 1 and 2 are only two illustrativeexamples of systems in which the present disclosure may be implemented.The present disclosure is in no way limited thereto. Extension of theprinciples of the present disclosure to systems having other dimensionswill be apparent to those skilled in the art. In particular, the numberof user terminals that will be present in a given implementation maydiffer, and may vary over time if they are mobile. The number ofantennas on the base station and user terminals is two in theillustrated example. More generally, any number, two or more, ofantennas can be employed such that MIMO communications are possible,though the number of receive antennas must be greater than to equal tothe number of data streams (i.e. layers) being transmitted. Both thebase station and user terminals include functionality not shown as wouldbe understood to one of skill in the art. Separate components are shownfor each of the MIMO channel 132, MIMO channel metric measurement module134,234, the CQI metric measurement module 136,236, and the compositefeedback module 138,238. More generally, the functions provided by thesemodules may be combined in one or more functional elements, and thesemay be implemented in one or a combination of software, hardware,firmware etc.

A MIMO system can be expressed as

${\overset{\omega}{y} = {{H\overset{\omega}{s}} + \overset{\omega}{\eta}}},$

where

$\overset{\varpi}{y} = \begin{bmatrix}y_{1} & y_{2} & K & y_{N}\end{bmatrix}^{T}$is a vector of communication signals received at a receiver;

$\overset{\varpi}{s} = \begin{bmatrix}s_{1} & s_{2} & K & s_{M}\end{bmatrix}^{T}$is a vector of communication signals transmitted by a transmitter.

$\overset{\varpi}{\eta} = \begin{bmatrix}\eta_{1} & \eta_{2} & K & \eta_{N}\end{bmatrix}^{T}$is a vector of noise components affecting the transmitted communicationsignals;

$H = \begin{bmatrix}h_{11} & h_{12} & K & h_{1M} \\h_{21} & h_{22} & K & h_{2M} \\M & M & O & M \\h_{N\; 1} & h_{N\; 2} & K & h_{NM}\end{bmatrix}$is a channel matrix of communication channel attenuation factors;

N is a number of antennas at the receiver; and

M is a number of antennas at the transmitter.

For a [2Tx, 2Rx] MIMO channel,

$H = \begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}$

The eigenvalue of H^(H)H are λmax, λmin. There are several schedulingapproaches, including orthogonality and capacity. These approaches arefor user terminal pairing only.

Where it is desired that scheduling by a BTS (such as BTS 100 and BTS200 in FIGS. 1 and 2 respectively) be provided on the basis of maximumorthogonality, the following MIMO channel metric will be computed by,for example, MIMO channel metric measurement module 134 in FIG. 1:

$\max\left\{ \frac{\det\left( {H^{H}H} \right)}{{trace}\left( {H^{H}H} \right)} \right\}$

The larger the metric, the more orthogonal is the channel.

For a maximum orthogonality decomposition scheduling scheme,

${\min{{\begin{bmatrix}h_{11} & h_{21}\end{bmatrix}\begin{bmatrix}* \\h_{12} \\* \\h_{22}\end{bmatrix}}}} = 0$

In this case, the channel is completely orthogonal, yielding twoseparate spatial channels, with channel attenuation factors being√{square root over (|h₁₁|²+|h₂₁|²)} and √{square root over(|h₁₂|²+|h₂₂|²)} respectively.

For scheduling based on a best conditional number of MIMO channelscheme, the following channel metric will be calculated by, for example,MIMO channel metric measurement module 134 in FIG. 1:ρ=λmax/λmin=˜1

In this case, an advanced receiver (maximum likelihood detection) and/ora simplified receiver can be employed.

For scheduling based on maximum capacity, the following metric will becalculated by, for example, MIMO channel metric measurement module 134in FIG. 1:det(H ^(H) H)

Maximum capacity scheduling is also maximum CQI scheduling.

For scheduling based on maximum orthogonality for several MIMO channels,

$H_{1} = \begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}$ $H_{2} = \begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}$

the following metric will be calculated by, for example, MIMO channelmetric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}\frac{\det\left( {H_{i}^{H}H_{i}} \right)}{{trace}\left( {H_{i}^{H}H_{i}} \right)}} \right\}$

For scheduling based on orthogonality capacity for several MIMOchannels, the following metric will be calculated by, for example, MIMOchannel metric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}{\det\left( {H_{i}^{H}H_{i}} \right)}} \right\}$

For scheduling based on a combined conditional number for several MIMOchannels, the following metric will be calculated by, for example, MIMOchannel metric measurement module 134 in FIG. 1:

$\min\left\{ {\sum\limits_{i = 1}^{2}\rho_{i}} \right\}$

For scheduling based on a SNR weighted maximum orthogonality scheme forseveral MIMO channels, the following metric will be calculated by, forexample, MIMO channel metric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}{{SNR}_{i}\frac{\det\left( {H_{i}^{H}H_{i}} \right)}{{trace}\left( {H_{i}^{H}H_{i}} \right)}}} \right\}$

For scheduling based on a SNR weighted capacity scheme for several MIMOchannels, the following metric will be calculated by, for example, MIMOchannel metric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}{{SNR}_{i}\mspace{14mu}{\det\left( {H_{i}^{H}H_{i}} \right)}}} \right\}$

For scheduling based on a SNR weighted combined conditional numberscheme for several MIMO channels, the following metric will becalculated by, for example, MIMO channel metric measurement module 134in FIG. 1:

$\min\left\{ {\sum\limits_{i = 1}^{2}{{SNR}_{i}\rho_{i}}} \right\}$

FIG. 3A illustrates a table setting out one representative example of alookup table for selecting coding modulation and MIMO modes based CQIwhich can be used in accordance with one embodiment of the presentdisclosure. The table in FIG. 3B provides a key for the table in FIG.3A.

In the table of FIG. 3A, Row 1 lists possible CQIs, which in this casewould be from 1 to 10. Each CQI has an associated FEC code, anassociated modulation, and an associated MIMO mode, in this case an STCcode. In the particular example, illustrated, there are five availableFEC codes, three available modulations, and two available STC codes.

The key shown in FIG. 3B indicates the code rates ⅕, ⅓, ½, ⅔, ⅘associated with the five available FEC codes, indicates the modulationconstellations 4-QAM, 16-QAM 64-QAM associated with the threemodulations, and indicates the STC modes STTD and BLAST associated withthe two available STC modes.

FIG. 3A shows how CQI, together with STC code rate, determines theproper code and modulation set. The table in FIG. 3A is of course justone possible example of a lookup table. The particular MIMO modes, FECcodes, and modulations supported will vary on an implementation specificbasis. The present disclosure is in no way limited thereto. Extension ofthe principles of the present disclosure to others possible lookuptables will be apparent to those skilled in the art. For example, otherforms of lookup tables could be employed which include other standardmodulation schemes such as Phase-shift keying (PSK), and other forms ofMIMO modes such as beamforming.

FIGS. 4A and 4B illustrate a lookup table in which the CQI and MIMOI arefed back in separate feedback components. In this case, the MIMO mode isdetermined by both CQI and MIMOI.

FIG. 4A sets out one example for coding modulation based on CQI. Row 1lists possible CQIs which may be calculated by CQI metric measurementmodule 136, from 1 to 10. Row 2 lists Forward Error Correction Codes(FEC), which in this case are from 1 to 5. Row 3 lists three possibleforms of modulation which can be implemented in accordance with thislookup table, namely 4-QAM, 16-QAM, and 64-QAM. Reference may be had tothe table in FIG. 3B which provides a key for the table in FIG. 4A.

FIG. 4B is a lookup table for MIMO modes based on a MIMO indicator(MIMOI). MIMO indicator may be calculated by MIMO channel metricmeasurement module 134 shown in FIG. 1. In this case, Row 1 listspossible MIMO indicators from 1 to 10 which are selected based on theresult of the channel metric calculations. Row 2 lists possible MIMOformats (which could represent, e.g. BLAST, STTD, beamforming, spatialmultiplexing, etc.) from 1 to 5.

FIGS. 4A and 4B show that the higher the CQI/MIMOI, the higher is themodulation order and FEC coding rates. There are two situations where acomposite metric of both CQI and MIMOI may be used: the first is MIMOmode selection, because when MIMOI is low, SM may not be used even witha large CQI. FIG. 8 shows that at low MIMOI, in the high CQI region, theuse of STTD or SM as a MIMO mode depends on MIMOI. However, in the sameCQI region, when MIMOI is larger, SM is selected. Another situation ismodulation and code set selection in SM, because in SM, MIMOI indicatesinter-layer interference, and hence indicates the performance of thechannel. Given the same CQI, modulation and code set will depends on theMIMO indicator. In other words, modulation and code set will bedetermined by two parameters: CQI and MIMOI.

As with the table in FIG. 3A, the tables in FIGS. 4A and 4B are merelyillustrative examples of lookup tables which could be used in accordancewith the present disclosure. Persons skilled in the art will appreciatethat other combinations of parameters can be employed.

FIG. 5 is a diagram illustrating another example of a lookup for MIMOmodes based on a MIMO indicator and a CQI. As noted above, a MIMOI maybe ascertained by calculations performed by MIMO channel metricmeasurement module 134 shown in FIG. 1 and CQI may be calculated by CQImetric measurement module 136 also shown in FIG. 1.

In FIG. 5, three sections are shown, a first section labelled “STTD”, asecond section labelled “spatial multiplexing (SM), and a third sectionlabelled “STTD/SM”. According to the lookup of FIG. 5, for low CQIs(i.e. low channel quality), regardless of the MIMO indicator, STTDshould be chosen as the MIMO format. For high MIMO indicators and highCQIs, the lookup of FIG. 5 indicates that spatial multiplexing should bechosen as the MIMO format. For a low MIMO indicator and high CQI, eitherof STTD and spatial multiplexing can be selected as the MIMO format.

For the example of FIG. 5, a CQICH (channel quality indicator channel)can be used to feedback coding/modulation information and/or selection,and a single bit can be used to flag the MIMO mode. One of ‘0’ or ‘1’can be used for STTD, and the other of ‘0’ or ‘1’ can be for SM.

FIG. 5 is of course just one possible example of a lookup in which thepresent disclosure may be implemented. The present disclosure is in noway limited thereto. Extension of the principles of the presentdisclosure to other lookup diagrams will be apparent to those skilled inthe art.

FIG. 6 is a plot of BLER (Block Error Rate) versus SNR (Signal-to-NoiseRatio) for an embodiment of the present disclosure. FIG. 6 shows theimpact on maximum capacity scheduling. FIG. 7 is a plot of BLER (BlockError Rate) versus SNR (Signal-to-Noise Ratio) for another embodiment ofthe present disclosure. FIG. 7 shows the impact on maximum capacityscheduling. Both FIGS. 6 and 7 shows two sets of curves, namely,“without scheduling” and “with scheduling”. More particularly, withoutscheduling” means “random scheduling”, and “with scheduling” means“orthogonality based scheduling”.

FIG. 8A is a plot of Cumulative Distribution Function (CDF) versuscondition number of H^(H)H for an embodiment of the present disclosure.H^(H)H means Hermitian transform, i.e. conjugate transposition. FIG. 8Apresents the impact on minimum conditional number scheduling. FIG. 8Ashows the CDF of the composite metric when random pairing is used (thesolid line) and when the orthogonality based pairing is used.

FIG. 8B is a plot of CDH versus (HHH) for another embodiment of thepresent disclosure. FIG. 8B presents the impact on minimum conditionalnumber scheduling.

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the presentdisclosure may be practised otherwise than as specifically describedherein.

What is claimed is:
 1. A method comprising: receiving a composite metricspecifying a channel quality indicator (CQI) of a multiple-inputmultiple-output Orthogonal Frequency Division Multiplexing (MIMO-OFDM)channel and a MIMO-OFDM channel indication to a receiver of thecomposite metric; and scheduling transmission to at least two userterminals based on the received composite metric, wherein the schedulingis orthogonality-based scheduling.
 2. The method of claim 1, wherein thecomposite metric is transmitted using a MIMO-OFDM transmission.
 3. Themethod of claim 1, further comprising transmitting pilot signalscorresponding to each of a plurality of antennas using a respective setof OFDM sub-carriers and OFDM symbol durations for each antenna.
 4. Themethod of claim 1, further comprising transmitting data to a userterminal, at least one of a modulation and coding of the data or a MIMOmode of the data being based on the received composite metric.
 5. Themethod of claim 1, further comprising receiving an indication of a MIMOcapability, the MIMO capability indicating a rank of the MIMO channel ora number of antennas on a user terminal from which a composite metric isreceived.
 6. A base station comprising: a receiver configured to receivea composite metric specifying a channel quality indicator (CQI) of amultiple-input multiple-output Orthogonal Frequency DivisionMultiplexing (MIMO-OFDM) channel and a MIMO-OFDM channel indication tothe base station, wherein the base station is configured to scheduletransmission to at least two user terminals based on the receivedcomposite metric, and wherein the scheduling is orthogonality-basedscheduling.
 7. The base station of claim 6, wherein the composite metricis transmitted using a MIMO-OFDM transmission.
 8. The base station ofclaim 6, further comprising a transmitter configured to transmit pilotsignals corresponding to each of a plurality of antennas using arespective set of OFDM sub-carriers and OFDM symbol durations for eachantenna.
 9. The base station of claim 6, further comprising atransmitter configured to transmit data to a user terminal, at least oneof a modulation and coding of the data or a MIMO mode of the data beingbased on the received composite metric.
 10. The base station of claim 6,wherein the receiver is further configured to receive an indication of aMIMO capability, the MIMO capability indicating a rank of the MIMOchannel or a number of antennas on a user terminal from which acomposite metric is received.
 11. A method comprising: receiving acomposite metric specifying a channel quality indicator (CQI) of amultiple-input multiple-output Orthogonal Frequency DivisionMultiplexing (MIMO-OFDM) channel and a MIMO-OFDM mode indication to areceiver of the composite metric; and receiving an indication of aMIMO-OFDM capability, the MIMO-OFDM capability indicating a number ofantennas on a user terminal from which the composite metric is received.12. The method of claim 11, wherein the composite metric is transmittedusing a MIMO-OFDM transmission.
 13. The method of claim 11, furthercomprising transmitting pilot signals corresponding to each of aplurality of antennas using a respective set of OFDM sub-carriers andOFDM symbol durations for each antenna.
 14. The method of claim 11,further comprising transmitting data to the user terminal, at least oneof a modulation and coding of the data or a MIMO-OFDM mode of the databeing based on the received composite metric, wherein the compositemetric comprises at least a first bit specifying the CQI and at least asecond bit specifying the MIMO-OFDM mode.
 15. A base station comprising:a receiver configured to receive a composite metric specifying a channelquality indicator (CQI) of a multiple-input multiple-output OrthogonalFrequency Division Multiplexing (MIMO-OFDM) channel and a MIMO-OFDM modeindication to the base station, wherein the receiver is furtherconfigured to receive an indication of a MIMO-OFDM capability, theMIMO-OFDM capability indicating a number of antennas on a user terminalfrom which the composite metric is received.
 16. The base station ofclaim 15, wherein the composite metric is transmitted using a MIMO-OFDMtransmission.
 17. The base station of claim 15, further comprising atransmitter configured to transmit pilot signals corresponding to eachof a plurality of antennas using a respective set of OFDM sub-carriersand OFDM symbol durations for each antenna.
 18. The base station ofclaim 15, further comprising a transmitter configured to transmit datato the user terminal, at least one of a modulation and coding of thedata or a MIMO mode of the data being based on the received compositemetric, wherein the composite metric comprises at least a first bitspecifying the CQI and at least a second bit specifying the MIMO mode.